AVES. ARCHAEORNITHES. 297 



metatarsal is nearly always free. The pes never has more than 

 four digits in the adult. 



The class Aves is most conveniently divided into two sub- 

 classes : 1. Archaeornithes. 2. Neornithes. 



Subclass I. ARCHAEORNITHES. 



The only form referred to this subclass of extinct birds is 

 Archaeopteryx\ the earliest known bird. In this animal the 

 skeleton does not seem to be pneumatic. The cervical and 

 trunk vertebrae are generally thought to be flat, certainly 

 their articulating surfaces are not saddle-shaped. There is no 

 long compound sacrum as in modern birds. The tail is longer 

 than the whole body, the caudal vertebrae are twenty in 

 number, they gradually taper as traced away from the trunk, 

 and each bears a pair of feathers. The posterior caudal 

 vertebrae are not united together to form a pygostyle. The 

 upper jaw bears thirteen pairs of conical teeth, planted in 

 distinct sockets in the maxillae and preinaxillae, but the 

 mandible has only three pairs. The presence of these teeth 

 forms the most essential difference between the skull of 

 Archaeopteryx arid that of modern birds, and the fact that 

 they occur on the premaxillae renders it improbable that 

 a horny beak was present. There is a ring of ossifications 

 in the sclerotic. The ribs do not show uncinate processes, 

 and articulate with the vertebrae by single heads not divided 

 into capitula and tubercula. Abdominal ribs appear to have 

 been present. The furcula is large, and the scapula has a well 

 developed acromion. The sternum is unknown. The radius 

 and ulna are approximately equal in size. In the manus the 

 first, second and third digits 2 are present, each terminated by 



1 E. Owen, Phil. Trans., vol. CLIII., p. 33; 1863. T. H. Huxley, 

 P. R. S., vol. xvi., p. 243; 1868. C. Vogt, Rev. Scient., ser. 2, torn. 9, 

 p. 241; 1879. C. H. Hurst, Nat. Sci., vol. in., p. 275; 1893; vol. vi., 

 pp. 112, 180, 244; 1895. W. P. Pycraft, Nat. Sci., vol. v., pp. 350 and 

 437; 1894; and vol. vm., p. 261; 1896. 



2 According to Hurst the fourth and fifth digits are also present. 



